A popular Disney hit re-imagined as a rock song was not a problem for Kat Perkins, who turned the catchy tune embraced by fans worldwide, "Let It Go," into a head-banging anthem on Monday night's The Voice semi-final.

"You murdered it," said an enthusiastic Usher of Perkins's powerful take on the song from Frozen, which she dedicated to the Keller children for whom she served as nanny, and who watched adoringly from the audience.

"I don't know what I can say anymore. It doesn't get better as far as vocals are concerned," noted her coach Adam Levine.

The somewhat controversial Twitter instant save returned for a second season, but with an added feature: The artists in the bottom three would have one last chance to save themselves by performing another song, just before another online vote.

The show kicked off calmly, as host Carson Daly revealed the names of the contestants who were moving on to next week, coach Shakira performed a single off her new CD, and coaches Adam Levine and Blake Shelton performed with their teams.

But things got interesting about two-thirds of the way through the hour-long show when it was revealed all of Levine and Shelton's artists were saved – what is their recipe for success? – leaving Team Usher and Team Shakira vulnerable for elimination.

"If there's any truth to the rumors, I'd just be really excited because I'm a huge fan," Levine told reporters after Monday night's live show kickoff. "She's a wonderful human being. I love her to pieces. So talented."

Host and producer Carson Daly also heaped praise on Stefani, but seemed to want to manage expectations about whether she'll be the one to replace Christina Aguilera.

"She would be great," Daly said. "These things are tough to work scheduling-wise. I don't know where we're at with it."